If, after running for many hours, it goes slow, what about quitting and restarting FCP X, or even restarting your Mac? Even 12GB may not be all that much if your projects are long and with lots of effects, and/or your project and event libraries are big. Also, try not to have nonnecessary applications running. They all eat up your RAM.
Take a look at Activity Monitor - I am guessing that free RAM getting too low may be the cause of the slowdown.
Whenever your free ram goes down into the tens of MB, your mac will slow down to a crawl.
Part of the problem is that OS X does not always release inactive RAM back to the free RAM pool as quickly as is needed. This is an OS problem that we're still hoping for a solution. It has become less serious in 10.8.2 but not completely solved, so one needs to be a little proactive in terms of RAM usage.
I will watch my Activity Monitor, but sometime problem starts if i only put some short (Gopro) files on the timeline, and all other projects are hidden (Event manager), also no effects or other gimmicks !!!!
Troubles started when I updated to Lion and again when I updated FCPX (I had to do this for the external monitoring with the Matrox Box)
And the fact that after a break fcpx is not responding seems to me very strange .
Luis here is an example what happens right now:
A complete clean start of FCPX, all old events and projects are hidden.
Import of 2o Min. SD !!! footage from a DVD. (Transcoded into h 264 with stream clip)
After some edits and 25 Seconds on the timeline total crash of the I mac !!!!!!
So what ??????????????????t This can not be a Ram Problem !!!!!!
Footage and Projekt on external Drive "LaCie" via thunderbolt.
I put the footage now on another external drive to test if "la Cie" is the reason for the troubles